Non-pasteurized, cold filtered draft beer is becoming an increasingly popular substitute for heat pasteurized beer. Beer is a complex liquid which contains a considerable amount of suspended solids and a number of organic constituents obtained from fermentation. Beer, being a complex solution, requires filtration in order to obtain a product of satisfactory quality. Commercial beer production generally involves multiple filtration steps where the first step is employed to remove larger particles and residues from the natural products associated with fermentation, "dirt." Often times a process may call for multiple dirt removal filtration steps for a particular type of beer. However, even following such steps the resulting filtrate is not necessarily safe for drinking since it may include an unacceptably high bacteriological titer.
For many years the industry has relied on heat pasteurization to reduce the amount of bacteria in draft beer to acceptable levels; those meeting governmental and industry standards. Heat pasteurization kills bacteria existing in the fermentation broth which is then subject to coarse filtration to remove relatively large suspended matter, e.g., 0.65 .mu.m and larger. However effective to reduce bacterial contamination, heat pasteurization forms many undesirable by-products which adversely affect the quality (taste) of the beer. Thus, the industry with increasing frequency is turning to cold filtration techniques, i.e., filtration occurring at about 33.degree. F.
In cold filtration processes, the application of heat is avoided which, in turn, avoids the formation of undesirable heat by-products. However, since heat is not employed to kill the ambient bacteria in raw beer ferment, filtration is relied on to provide the requisite degree of bacterial removal, at least to a titer satisfying the various mandatory standards. For example, the titer of lactose bacillus must be reduced significantly in order to meet such standards. To achieve such removal, the filter should have at least a 0.45 .mu.m rating which contrasts with the coarser 0.65 .mu.m rating for heat pasteurized beer.
In order to provide a reasonable degree of processing flexibility in selecting filtration parameters, the beer industry has turned to, among other things, supported polymeric membranes for cold beer filtration. Such membranes, such as those formed from polyvinylidene fluoride (PVDF) are particularly suited for cold beer filtration since the performance ratings are easily modified and the filters can be employed with typical batch processing equipment used for beer fermentation.
Thus, by departing from heat pasteurization and favoring cold filtering, the filters must provide increased performance criteria. Accordingly, cold filtration processes involve a higher degree of filtration to produce an effective bacterial removal rating in order to meet rigorous governmental pure food standards and sometimes more stringent industry standards while preserving the taste and quality of the beer.
The significant problem associated with the finer filters required for cold beer filtration is that they clog more easily than their coarser counterparts used for heat pasteurized beer. Clogging is due to protein and polysaccharide loading. Accordingly, supported polymeric membranes for cold beer filtration must either be disposed of or cleaned for re-use of the filters. To clean the filter membrane requires removal of proteins and polysaccharides sorbed in the membrane pores which cause membrane loading and eventual undesirable pressure rise in the filtering apparatus. Loading of chill haze constituents, e.g., proteins and polysaccharides occurs when such molecules which are smaller than the pore size of the particular filter but aggregates in the pores and thereby reduce the effective flow rate and uniformity of flow within the filter. In order to remove such loaded proteins and polysaccharides, techniques have been developed to clean such membranes and de-absorb and eliminate such macromolecules.
Commonly, the industry has turned to enzymes and detergents for cleaning PVDF (polyvinylidene fluoride) filters. Cleaning such filters relies on various oxidizers employed in the enzyme or detergent. PVDF membranes are generally expensive and more difficult to manufacture and utilize than, for example, polyamide (nylon) membranes. However, to date, polyamide (nylon 66) membranes, while being less expensive, have been shunned for cold draft beer filtration, since commonly employed microporous membrane cleaning techniques, i.e., enzymes and detergent, are generally inapplicable and have proven unsuccessful with polyamide membranes. Accordingly, once polyamide membrane filter reaches a terminal .DELTA.p (pressure drop), the filter cartridges must be removed from the filtration tank, replaced, and the spent cartridges disposed of. This is both labor intensive and unduly costly.
The physical characteristics, apart from the cleaning problem, of polyamide membranes make them ideally suited for cold beer filtration since polyamide membranes provide process control flexibility, resist pressure deformation, provide uniform filtration characteristics for reducing bacteriological titers to satisfactory levels, are reliable and relatively inexpensive, and are easily adapted for different filter configuration. Furthermore, such polyamide membranes exhibit solution shape stability, substantially uniform flow characteristics, and substantially uniform bacteriological removal, i.e., pore ratings. An exemplary water-wettable, alcohol-insoluble, skinless, cast, polyamide membrane of the type for use in the instant invention is disclosed in U.S. Pat. No. 4,340,479, issued to Pall et al., and which is incorporated by reference herein. The polyamide (Nylon 66) membrane exhibits excellent performance characteristics applicable for use in cold draft beer filtration process. However, as noted, these polyamide membrane filters have not lent themselves to effective cleaning using detergents and enzymes; the commonly employed beer filter cleaning agents.
Microporous membranes conventionally when used for large scale cold draft beer filtration are generally disposed in an array fixed in a used filtration tank filter apparatus. Such tank filters may include an array of, for example, 80 forty inch filter cartridges. A tank of this type can be characterized by a filter assembly with an upper housing portion and a lower housing portion hinged to one another. The lower housing portion contains a flanged inlet port and a flanged drain port opening directly to a chamber. A centrally disposed standpipe in the chamber is in fluid communication with a flanged outlet port formed in the lower chamber and extending upwardly to and above the plane of separation between the upper and lower housing portions of the chamber. An easily accessible and replaceable, removable, sealed, filter array support with an array of stacked membrane filter cartridges, is positioned within the housing portions. The array support includes a lower perforated support plate and an upper, fluid impervious "tube sheet" or separation sheet, including a gasketed central opening dimensioned to complement the standpipe, is disposed and sealed between the upper and lower housing portions to provide a fluid tight seal therebetween.
In operation, the drain is blocked and the fluid, in this case, raw draft beer, is pumped into the lower housing portion. The fluid is confined, under pressure, in the lower chamber by the "tube sheet" and is constrained to flow only through the membrane filter elements. Thus, the beer passes from the outside of the cartridges to the inside of the cartridges through the microporous membrane and into the upper housing portion. The filtered beer collects in the upper housing portion and is removed, by gravity draining, through the outlet port in fluid communication with the standpipe.
The above arrangement is particularly useful since the filter array is cleanable either in place, or by opening the assembly by pivoting the upper and lower housing along the hinge and removing the filter array. If cleaned in place, the inlet source for raw beer is closed, and a cleaning fluid is introduced into the lower chamber. Sludge can be drained from the lower chamber drain as well as excess cleaning solution. The cleaning and rinsing fluid which passed through the filter cartridge to the upper chamber is drained from the assembly through the standpipe.
The beer brewing industry, commonly cleans brewing and processing equipment with a dilute caustic (NaOH) solution at elevated temperatures. Conventional membranes employed in beer filtration such as PVDF are not compatible with caustic solutions, i.e., the membranes will be damaged. Therefore, the membrane cartridges are either discarded or special provisions must be made to clean the filter cartridge units, alone apart from the tank and other filtering apparatus.
Finally, due to the inherent variations in different beers, there is no one uniform standard for filters used in cold beer filtration. Starting products are not identical, the bacterial involved in the fermentation and beer manufacturing process are not identical, and the final filtered beers are not identical. Accordingly, filter requirements differ from beer-to-beer since they are governed by the size of the bacteria which, in turn, are governed by the particular process employed for a particular beer. Thus, the performance characteristics and requirements for filters associated with cold draft beer filtration processes are not universal.